Generally, one of two modulation schemes is used for high accuracy Interferometric Fiber Optic Gyroscope (IFOG): Square-wave modulation or Dual Ramp modulation. Dual Ramp modulation allows the Vpi value of an Integrated Optics Chip (IOC) of the IFOG to be servoed, which provides a highly accurate scale factor. The expense of this modulation scheme is that it produces a glitch pattern at the front-end that has Eigen-frequency first harmonic content. This has the undesirable effect of introducing additional bias into the IFOG. The Square-wave modulation scheme provides a benign glitch pattern at the front-end containing primarily Eigen-frequency second harmonic content, which the IFOG is immune to. However, the Square-wave modulation does not provide a means for servoing Vpi.
FIGS. 1-1 and 2-1 are plots of two prior art bias modulation waveforms and their resulting glitch patterns. These bias modulation waveforms are currently used in high precision IFOG applications.
FIGS. 1-2 and 2-2 illustrate frequency content of the glitch patterns shown in FIGS. 1-1 and 2-1. The Square-wave modulation glitch pattern does not have any frequency spikes at the odd Eigen-frequency harmonics and, therefore, will (ideally) contribute no bias due to glitch pick-up during Front-End sampling. The Dual Ramp modulation glitch pattern shows substantial odd harmonic content and will contribute significant bias error in the absence of adequate glitch masking.
Another advantage of Dual Ramp modulation over Square-wave modulation is the decrease in Eigen-frequency content over the IOC drive waveform when certain modulation depths are used. Two commonly used modulation depths were investigated here (π/2 and 3π/4). FIGS. 5-1, 5-2, 6-1, and 6-2 show the frequency content for π/2 and 3π/4 modulation depths, respectively. As shown in FIGS. 5-1 and 5-2, at π/2 modulation depth the Dual Ramp modulation waveform has the same 1st harmonic Eigen-frequency content as Square-wave modulation. As shown in FIGS. 6-1 and 6-2, the Dual Ramp modulation waveform now has three times less first harmonic Eigen-frequency content than the Square-wave modulation. But the present invention has nine times lower signal level than the Square-wave modulation. These results show that the modulation scheme of the present invention is more favorable than both Square-wave and Dual Ramp modulation, from the perspective of electrical coupling between the IOC drive and Front-End electronics.